This invention relates to a strand tension controller. The invention has application in many types of strand processes where the strand is moving and the tension on the moving strand affects some downstream process being carried out on the strand. The embodiment disclosed in this application relates to a yarn tension controller useful for controlling tension on textile yarn processing machines such as, for example, winders and twisters. Processing yarn on these types of machines require careful control of tension because of the effect of yarn tension on dyeing characteristics and the ability of the yarn to properly react to processing steps such as air entangling and heat setting. Scientific study suggests that there is an ideal tension at which any particular yarn should be delivered to any given process. The closer to that ideal the actual processing approaches, the better the processed yarn. Scientific study has also demonstrated that the more uniform the tension at any given desired tension level, the more uniform the processed yarn.
Numerous types are devices are known for controlling yarn tension. These include devices as simple as merely applying weights such as metal disks to guides through which the yarn passes to add sufficient tension to dampen tension variations.
U.S. Pat. No. 3,937,417 discloses a yarn tension apparatus which accepts "essentially tensionless yarn" and progressively adds tension over a series of relatively long surfaces to "accommodate the abrupt fluctuations associated with tangle release events." As is made clear in this patent, the device is directed more towards adding tension and dampening any abrupt variations in the tension than in providing a uniform predetermined output tension on the yarn, even though a claim of uniform yarn tension is made. See, col. 4, 1. 13-16.
The Schurich U.S. Pat. No. 2,981,497 discloses a thread tensioner which includes a dancer arm which takes up slack in thread caused by the reciprocating movement of a hand-operated knitting machine. The dancer arm includes a pivot and a pair of brake plates adjacent the thread input end of the dancer arm which moves downward at its free end when a thick place, such as a knot, passes between the brake plates. The downward movement of the dancer arm weakens the braking action so that the knot can slip through more easily. Of course, the tension on the yarn is affected. It is clear, however, that major result is to vary the tension on the thread backwards through the thread path, not forward towards the knitting machine. While the slack may be taken up by the dancer arm movement, uniform tension is not achieved. Rather, the oscillation of the tension is dampened.
This is a subtle but important point. The object in controlling tension is to present to the yarn processing station a desired tension, and to present that desired tension uniformly on a real time basis. "Average" tension over time does not result in a suitable quality yarn over time. Yet most prior art tension devices are concerned mainly with dampening the amplitude of yarn variation by working backwards through the system to produce a yarn with a suitable "average" tension.
The invention disclosed in this application is the result of mathematical analysis which demonstrates that with the proper lever ratio tension can be "subtracted" from a pre-set tension working forward through a yarn path to output to a yarn processing station such as a heat setting device a yarn having a uniform, desired tension.
As a further refinement on the principle of outputting a uniformly tensioned yarn at a single station, a method and apparatus for controlling the uniformity and tension level of every yarn on a yarn processing machine has been devised and is also disclosed.